The organic electroluminescent device exhibits a bright application prospect in the display and illumination fields since it has the advantages, such as a simple manufacturing process, a wide range of materials, a controllable structure, being super-thin and flexible, etc. Some organic electroluminescent devices industrially applicable or close to industrially applicable have appeared, for example, display screens based on an active matrix organic electroluminescent diode from the Samsung company have been widely used in Galaxy series mobile phones and the large size televisions thereof also have appeared on the market. The organic white-light illumination technology also has been developing towards the target of low-cost, high-efficiency, and long-life flat-panel light sources.
In the organic fluorescent electroluminescent device invented by C. W. Tang et al., (U.S. Pat. No. 4,769,292; C. W. Tang et al., Appl. Phys. Lett 51, 913 (1987)), an organic fluorescent light-emitting material is used for luminescence in 1987. Under the application of an electric field, holes and electrons are injected from an anode and a cathode of a device, respectively, and they meet in a light-emitting area, forming excitons which emit light by radiative recombination. In the context of random spin directions of the electrons and holes injected, the resulting ratio of the singlet excitions to the triplet excitions is 1:3; for an organic fluorescent material, the transition of a triplet excition to the ground state is forbidden, there is only luminescence from the transition of a singlet excition to the ground state; therefore the internal quantum efficiency of the device has an upper limit of 25%.
In order to break through the upper limit of 25% of the internal quantum efficiency in an organic fluorescent electroluminescent device, M. E. Thompson et al., and Yuguang Ma. et., have invented organic phosphorescent electroluminescent devices (i.e., a device in which an organic phosphorescent material is used for luminescence, U.S. Pat. No. 6,303,238; M. A. Baldo, et al., Nature 395, 151, (1998); Y. G. Ma, et al., Synth. Met. 94, 245, (1998)). They incorporate heavy metal atoms in the organic light-emitting materials, i.e., the organic phosphorescent materials, wherein the transition of a triplet excition to the ground state becomes possible by the strong spin-orbital coupling of the heavy metal atom. For the organic phosphorescent electroluminescent device, the internal quantum efficiency can reach 100%. However, the current high-efficiency organic phosphorescent materials are all expensive rare metal coordination complexes, resulting significant increase of the production costs of organic phosphorescent electroluminescent devices.
In the organic florescent electroluminescent devices, the light emission of the singlet excitions are used (the upper limit of the internal quantum efficiency is only 25%); in the organic phosphorescent electroluminescent device, the light emission of the triplet excitions are used (the organic phosphorescent light-emitting materials are expensive); they have disadvantages of their own; therefore it is a trend to develop an organic electroluminescent device which can reach an upper limit of internal quantum efficiency of 100% without using expensive rare metals. So far there is no report on an organic electroluminescent device using a neutral free-radical electroluminescent material as a light-emitting layer.
Contents of Invention
An object of the present invention is to provide an organic electroluminescent device, which emits light on the basis of the transition of a doublet electron between different orbits of organic neutral free-radical electroluminescent materials, can replace an organic phosphorescent electroluminescent device. The device based on organic neutral free-radical electroluminescent materials has relatively low costs and has an internal quantum efficiency upper limit of 100%.
The neutral free-radical electroluminescent materials described in the present invention are classified into the following four classes: 1,3-bis(diphenylene)-2-phenylallyl free radicals and derivatives thereof; tri(2,4,6-trichlorophenyl)methyl free radicals and derivatives thereof; 3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl free radicals and derivatives thereof; (2,4,6-trichloro-5-pyrimidinyl)bis(2,4,6-trichlorophenyl)methyl free radicals and derivatives thereof, and have the following general structures:

wherein R, R1, R2 and R3 are the same or different,
R, R1, R2, R3═H, F, Cl, CN . . .

wherein R, R1, R2 and R3 are selected from an aromatic amine, heterocyclic aromatic hydrocarbon or fused ring aromatic hydrocarbon compound or an atom, a group as shown in the above formulas or a derivative from any combination thereof.
During the preparation of an organic electroluminescent device, the above-mentioned neutral free-radical electroluminescent material can be made as a light-emitting layer of the device by evaporation, etc.
Comparing with the Prior Art, the Present Invention has the Following Advantages:
(1) since there is only one electron in the outermost molecular orbital of an neutral free-radical electroluminescent material, the spin quantum number of one electron is ½, corresponding to a doublet state. The light emission of the organic electroluminescent device in the present invention is from the transition of the doublet electron of the organic neutral free-radical molecules from a higher level molecular orbit to a lower level singly occupied molecular orbit (SOMO), which is different from the emission from a singlet excition or a triplet excition;
(2) an electron from the outermost molecular orbit (SOMO) of the organic neutral free-radical electroluminescent material is excited to a higher level molecular orbit, and when the electron transits downward, it faces an empty molecular orbit, thus there is no limitation of Pauli-exclusion-forbidden transition; therefore the device using the emission of neutral free-radical molecules has an upper limit of the internal quantum efficiency of 100%.